Elemental Composition Analysis of Processed and Unprocessed

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Elemental Composition Analysis of Processed and Unprocessed Diesel Fuel by Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Christine A. Hughey,† Christopher L. Hendrickson, Ryan P. Rodgers, and Alan G. Marshall*,† Center for Interdisciplinary Magnetic Resonance, National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310 Received February 8, 2001. Revised Manuscript Received May 31, 2001

High-resolution (100 000 < m/∆m50% < 250 000, in which ∆m50% denotes mass spectral peak full width at half-maximum height) electrospray ionization Fourier transform ion cyclotron resonance positive-ion mass spectra of unprocessed (and processed) diesel fuels resolves approximately 500 (and 200) chemically different constituents over a mass range from 200 to 452 Da, with as many as 6 resolved elemental compositions at a given nominal mass. Molecular formulas were assigned from accurate mass measurement to within (1 ppm. Compound types were identified by Kendrick mass analysis. On the basis of the experimental behavior of model compounds, electrospray ionization was found to ionize selectively basic pyridine homologuess compounds responsible for deactivation of hydrotreatment catalysts and instability of fuels during storage. Compound classes identified in the unprocessed diesel fuel include those containing N, N2, NS, NO, N2O, O2, and SO and, in the processed diesel, N, N2, NO2, and SO. Comparison of unprocessed and processed diesel fuel reveals that N- and N2-type compounds are resistant to laboratory hydrotreatment. In contrast, NS-, NO-, N2O-, and O2-containing compounds were completely removed by hydrotreatment. Concentration-dependent dimers observed above 450 Da were confirmed by infrared multiphoton dissociation.

Introduction Advances in drilling and processing technology have allowed the petroleum industry to turn to heavier crude oils to meet consumer needs. Heavier crudes are more complex in composition (higher boiling point) and contain a higher percentage of heteroatomic (S, O, and N) hydrocarbons.1 Heteroatomic compounds contribute to fuel instability during storage2,3 and to air pollution upon combustion by release of NOx and SOx gases. To meet stringent environmental regulations and to produce a quality product, it is necessary to remove heteroatoms by hydrotreatment. Hydrotreatment involves the saturation of the hydrocarbon feedstock by H2 gas, followed by C-S or C-N fission and release of the heteroatom in inorganic form such as H2S or NH3.4-9 Unfortunately, complete removal of heteroatoms is difficult. Basic nitrogen-containing heteroatomic hydrocarbons (i.e., pyridine homologues) are particularly resistant to hydrotreatment.10,11 Their presence also reduces the efficiency of S and O removal because nitrogen com* To whom correspondence should be addressed. † Members of the Department of Chemistry, Florida State University. (1) Altgelt, K. H.; Boduszynski, M. M. Composition and Analysis of Heavy Petroleum Fractions; Marcel Dekker: New York, 1994. (2) Chmielowiec, J.; Fischer, P.; Pyburn, C. M. Fuel 1987, 66, 13581363. (3) Worstell, J.; Daniel, S. R.; Frauenhoff, G. Fuel 1981, 60, 485487.

pounds have been shown to deactivate the hydrotreatment catalyst through coke formation on the catalyst surface.12,13 Molecular identification of basic nitrogen heteroatomic species is important for design of better catalysts and, hence, more efficient refining processes. However, analysis/identification of basic nitrogen compounds in crude oils and petroleum distillates is made difficult by the complexity of the predominantly hydrocarbon matrix and the low concentration of nitrogen compounds in that matrix. Nitrogen concentration averages only 0.1-2 wt % in crude oils and parts per million level in diesel fuel.14 Basic nitrogen, determined by nitrogen-specific15-17 or flame ionization detector10,18 coupled to a gas chromatograph, infrared spectroscopy,11,19 or mass spectroscopy (see below), accounts for roughly one-third of the total nitrogen.14 To isolate and concentrate basic nitrogen compounds, time-consuming separation schemes prior to analysis have been necessary.18-22 Typically, these separation schemes employ (4) Angelici, R. J. Polyhedron 1997, 16, 3073-3088. (5) Callejas, M. A.; Martinez, M. T. Energy Fuels 1999, 13, 629636. (6) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 20212058. (7) Kabe, T.; Ishihara, A.; Qian, W. Hydrodesulfurization and Hydrodenitrogenation: Chemistry and Engineering; Wiley-VCH: Tokyo, 1999. (8) Prins, R.; Jian, M.; Flechsenhar, M. Polyhedron 1997, 16, 32353246. (9) Weller, K. J.; Fox, P. A.; Gray, S. D.; Wigley, D. E. Polyhedron 1997, 16, 3139-3163.

10.1021/ef010028b CCC: $20.00 © 2001 American Chemical Society Published on Web 07/10/2001

Elemental Composition Analysis of Diesel Fuel

liquid-liquid and/or solid-liquid extraction, both of which exhibit poor selectivity, incomplete separation, and other experimental complications (e.g., formation of emulsions, inapplicability to viscous crude oils, etc.).17,18 Moreover, certain heteroatomic compounds, once removed from the petroleum matrix, may oxidize. The complexity of petroleum mixtures also makes molecular identification difficult. The nitrogen fraction in gas oils, diesel feedstocks, and crude oils, even after isolation from the hydrocarbon matrix, consists of hundreds to thousands of compounds. Gas chromatographic analysis often yields simply an unresolved “hump”. Even if resolved, unambiguous molecular identification requires coelution of analyte and reference compounds on multiple columns or derivatization.10 Moreover, many of the components in petroleum products are unknown or, if known, are not commercially available as reference compounds. Gas chromatography/ mass spectrometry (GC/MS) simplifies identification by the use of mass measurement and characteristic fragmentation patterns but nevertheless suffers from lack of reference compounds as well as limited chromatographic and mass resolution.10,15-17,23 Prior to the advent of electrospray ionization (ESI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry, low-voltage electron ionization (EI) high-resolution mass spectrometry was the most successful tool for identification of heteroatomic compounds. Molecular formulas and Z types (Z is the “hydrogen deficiency” relative to alkanes, CcH2c+zX, in which X denotes heteroatomsssee below) have been assigned for some nitrogen, oxygen, and sulfur compounds present in middle distillates24 and crude oil distillation cuts.19,21,25,26 Although low-voltage EI with a doublefocusing sector mass analyzer yields higher resolving power than GC/MS (typically conducted with a quadrupole mass analyzer), prechromatographic separation of nitrogen and hydrocarbon fractions is still necessary to avoid the common interference between 14N and 13CH (0.00815 Da mass difference). Alternatively, that interference may be avoided by performing ammonia chemical ionization high-resolution mass spectrometry (10) Schmitter, J. M.; Ignatiadis, I.; Dorbon, M.; Arpino, P.; Guiochon, G.; Toulhoat, H.; Huc, A. Fuel 1984, 63, 557-564. (11) Holmes, S. A.; Thompson, L. F. Fuel 1983, 62, 709-717. (12) Nagai, M.; Kabe, T. J. Catal. 1983, 81, 440-449. (13) La Vopa, V.; Satterfield, C. N. J. Catal. 1988, 110, 375-387. (14) Speight, J. G. The Desulfurization of Heavy Oils and Residua; 2nd ed.; Marcel Dekker: New York, 2000. (15) Shin, S.; Sakanishi, K.; Mochida, I. Energy & Fuels 2000, 14, 539-544. (16) Wiwel, P.; Knudsen, K.; Zeuthen, P.; Whitehurst, D. Ind. Eng. Chem. Res. 2000, 39, 533-540. (17) Merdrignac, I.; Behar, F.; Albrecht, P.; Briot, P.; Vandenbroucke, M. Energy Fuels 1998, 12, 1342-1355. (18) Schmitter, J. M.; Ignatiadis, I.; Arpino, P.; Guiochon, G. Anal. Chem. 1983, 55, 1685-1688. (19) McKay, J. F.; Weber, J. H.; Lantham, D. R. Anal. Chem. 1976, 48, 891-898. (20) Green, J. B.; Hoff, R. J.; Woodward, P. W.; Stevens, L. L. Fuel 1984, 63, 1290-1301. (21) Snyder, L. R.; Buel, B. E.; Howard, H. E. Anal. Chem. 1968, 40, 1303-1317. (22) Hsu, C. S.; Qian, K.; Robbins, W. K. J. High Resolut. Chromatogr. 1994, 17, 271-276. (23) Walls, C. L.; Beal, E. J.; Mushrush, G. W. J. Environ. Sci. Health 1999, A34, 31-51. (24) Severin, D.; David, T.; Brouwer, L. Pet. Sci. Technol. 1999, 17, 1043-1049. (25) Snyder, L. R. Anal. Chem. 1969, 41, 314-323. (26) Snyder, L. R. Anal. Chem. 1969, 41, 1084-1094.

Energy & Fuels, Vol. 15, No. 5, 2001 1187

so that nitrogen compounds are ionized selectively.22,27,28 Even so, prechromatographic separation is generally employed, and mass resolving power remains inadequate for analysis of heavy petroleum. The coupling of microelectrospray ionization with high-field (9.4 T) FT-ICR MS makes possible the complete identification of basic nitrogen compounds in complex petroleum mixtures without prior chromatographic isolation. Positive-ion electrospray ionization selectively ionizes basic nitrogen compounds for mass analysis.29 Furthermore, ESI can ionize high molecular weight species inaccessible by thermal vaporization followed by electron ionization. FT-ICR MS routinely provides mass resolving power, m/∆m50% > 100 000 (in which ∆m50% denotes mass spectral peak full width at half-maximum height) and mass accuracy 100 000) and high mass accuracy (1 ppm) mass spectra allow for resolution of ∼800 elemental compositions between 200 and (27) Veloski, G. A.; Lynn, R. J.; Sprecher, R. F. Energy Fuels 1997, 11, 137-143. (28) Buchanan, M. V. Anal. Chem. 1982, 54, 570-574. (29) Qian, K.; Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G. Anal. Chem. 2001, 15, 492-498. (30) Gough, M. A.; Langley, G. J. Rapid Commun. Mass Spectrom. 1999, 13, 227-236. (31) Zhan, D.; Fenn, J. B. Int. J. Mass Spectrom. 2000, 194, 197208. (32) Miyabayashi, K.; Suzuki, K.; Teranishi, T.; Naito, Y.; Tsujimoto, K.; Miyake, M. Chem. Lett. 2000, 172-173. (33) Rodgers, R. P.; Hendrickson, C. L.; Emmet, M. R.; Marshall, A. G.; Greaney, M.; Qian, K. Can. J. Chem. 2000, in press.

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Table 1. Properties and Composition of the Raw Diesel Feedstock and Processed Diesel Samples raw diesel feedstock 50 wt % light cycle oil from a fluid catalytic cracker 50 wt % straight run light gas oil specific gravity (60/60 °F) ) 0.8820 composition H (wt %) 12.1 C (wt %), 87.4 S (wt %), 1.39 N (wt ppm), 392 boiling range (ASTM D86) IBP, 193 °C 10%, 232 °C 30%, 261 °C 50%, 289 °C 70%, 319 °C 90%, 358 °C 95%, 372 °C processed diesel sample composition S (wt %), 0.037 N (wt ppm), 140 laboratory hydrotreatment conditions catalyst, CoMo (cobalt-molybdenum) temperature, 334 °C hydrogen pressure, 29.6 atm liquid hourly space velocity, 2.0 h-1 hydrogen-to-oil ratio, 500 NL/L (Normal liter/liter)

800 Da and identification of ∼500 elemental compositions (those below 450 Da) on the basis of accurate mass measurement alone. Comparison of mass spectra obtained before and after hydrotreatment establishes the efficiency of basic nitrogen heteroatom removal during processing. Experimental Section Sample Preparation. The raw diesel fuel sample obtained from Haldor Topsoe A/S was composed of 50% light cycle oil from a fluid catalytic cracker and 50% straight run light gas oil. Various properties of the raw diesel sample are listed in Table 1. The processed diesel fuel sample was obtained from a laboratory hydrotreatment reactor. Both diesel samples were used as received. A ∼10% solution of each was prepared separately by dissolving 100 µL of diesel in 1 mL of methylene chloride. Prior to analysis by ESI-FT-ICR MS, a solution of each diesel sample was prepared by dissolving 50 µL of the ∼10% diesel/methylene chloride solution in 200 µL of acetonitrile spiked with 0.3% acetic acid. All solvents were pesticide grade or better and obtained from Fisher Scientific. A 20 µM mixture of model nitrogen compounds was prepared in acetonitrile spiked with 0.3% acetic acid to determine whether basic nitrogen compounds were indeed selectively ionized by electrospray. The mixture consisted of acridine, oxindole, 1-aminoanthracene, 2-hydroxyquinoline, 1,10-phenanthroline, and carbazole in equal concentrations. Butyl sulfoxide and phenyl sulfoxide were also used as model compounds because SO-containing compounds were detected in both raw and processed diesel fuels. All model compounds were obtained from Aldrich Chemical Co. Mass Analysis. Mass analyses were carried out with a home-built FT-ICR mass spectrometer34 equipped with a 22 cm horizontal room-temperature bore 9.4 T

Hughey et al.

magnet (Oxford Corp., Oxney Mead, England). Data were collected and processed by a modular ICR data acquisition system (MIDAS).35 Positive ions were generated from a microelectrospray source equipped with a 50 µm id fused silica micro ESI needle.36 Samples were infused at a flow rate of 400 nL/min. Typical ESI conditions were: needle voltage, 1.8 kV; tube lens, 390 V; and heated capillary current, 3 A. Ions were accumulated externally in a linear octopole ion trap for 8-12 s and transferred through rf-only multipoles to a 10 cm diameter, 30 cm long open cylindrical Penning ion trap.37 Multipoles were operated at 1.75 MHz at a peak-to-peak rf amplitude of 170 V. Broadband frequency chirp dipolar excitation (70 kHz to 1.27 MHz at a sweep rate of 150 Hz/µs and peak-to-peak amplitude, 190 V) was followed by direct mode image current detection that yielded 2 Mword time-domain data. One hundred coadded time domain data sets were Hanningapodized, followed by a single zero-fill before fast Fourier transformation and magnitude calculation. Frequency was converted to mass-to-charge ratio by the quadrupolar electric trapping potential approximation38,39 to generate the spectra shown in Figures 1-8. To ensure the absence of noncovalent ion complexes (i.e., dimers at higher m/z), infrared multiphoton dissociation (IRMPD) was performed with a continuous wave 40 W CO2 laser (Synrad E48-2-155, Bothell, WA). Mass Calibration, Kendrick Mass Series, and Elemental Composition Assignments. All spectra were internally calibrated against poly(ethylene glycol) (PEG) 200, 400, or 600 (Aldrich Chemical Co.). The mass values for ions of 200 < m/z < 750 and relative abundances greater than 4% were imported into an Excel spreadsheet. Measured masses were converted from the IUPAC mass scale (12C ) 12.000 00 Da) to the Kendrick mass scale (CH2 ) 14.000 00 instead of 14.015 65) according to eq 140

Kendrick Mass ) IUPAC mass × (14/14.01565) (1) Kendrick mass is rounded up to the nearest integer to give nominal Kendrick mass. Nominal Kendrick masses were then sorted into even and odd values by use of an Excel macro and the Kendrick mass defect (KMD) calculated from eq 2

KMD ) (Nominal Kendrick Mass Kendrick Mass) × 1000 (2) As discussed below, conversion to Kendrick mass and the calculation of the KMD simplifies the identification of homologous series (namely, compounds having the same number of rings, double bonds, and heteroatoms but differing in number of CH2 groups) because mem(34) Senko, M. W.; Hendrickson, C. L.; Pasa-Tolic, L.; Marto, J. A.; White, F. M.; Guan, S.; Marshall, A. G. Rapid Commun. Mass Spectrom. 1996, 10, 1824-1828. (35) Senko, M. W.; Canterbury, J. D.; Guan, S.; Marshall, A. G. Rapid Commun. Mass Spectrom. 1996, 10, 1839-1844. (36) Emmett, M. R.; White, F. M.; Hendrickson, C. L.; Shi, S. D.H.; Marshall, A. G. J. Am. Soc. Mass Spectrom. 1998, 9, 333-340. (37) Senko, M. W.; Hendrickson, C. L.; Emmett, M. R.; Shi, S. D.H.; Marshall, A. G. J. Am. Soc. Mass Spectrom. 1997, 8, 970-976. (38) Ledford, E. B. J.; Rempel, D. L.; Gross, M. L. Anal. Chem. 1984, 56, 2744-2748. (39) Shi, S. D.-H.; Drader, J. J.; Freitas, M. A.; Hendrickson, C. L.; Marshall, A. G. Int. J. Mass Spectrom. 2000, 195/196, 591-598. (40) Kendrick, E. Anal. Chem. 1963, 35, 2146-2154.



Table 2. Kendrick Masses for Z Series, -11N measured mass (Da)

relative abund (%)

Kendrick mass

theoretical Kendrick mass

error (mDa)

Kendrick mass defect

molecular formula (M + H)+

256.2065 270.2222 284.2378 298.2534 312.2691 326.2847 340.3003 354.3160 368.3313 382.3469 396.3625 410.3781 424.3939 438.4101

11.301 27.588 51.968 77.837 92.832 100.000 93.617 69.852 49.621 36.463 24.870 15.780 8.943 3.975

255.9205 269.9205 283.9204 297.9204 311.9204 325.9204 339.9203 353.9204 367.9201 381.9200 395.9199 409.9199 423.9201 437.9205

255.9204 269.9205 283.9204 297.9205 311.9204 325.9205 339.9204 353.9205 367.9204 381.9205 395.9204 409.9205 423.9204 437.9205

+0.1 0.0 +0.0 -0.1 0.0 -0.1 -0.1 -0.1 -0.3 -0.5 -0.5 -0.6 -0.3 0.0

80 80 80 80 80 80 80 80 80 80 80 80 80 79

C18H26N C19H28N C20H30N C21H32N C22H34N C23H36N C24H38N C25H40N C26H42N C27H44N C28H46N C29H48N C30H50N C31H52N

bers of the same homologous/Kendrick series have the same KMD. Sorting by KMD in Excel groups Kendrick series together (see Table 2). Once a given homologous series is identified, molecular formulas are assigned on the basis of accurate mass in the range 200 < m/z < 452 for both the raw and the processed diesel samples. Elemental compositions are assigned by use of a mass calculator program limited to molecular formulas consisting of up to 100 12C atoms, up to 2 13C, up to 200 1H, up to 3 14N, up to 5 16O, up to 5 32S, and up to 1 34S. The mass tolerance is set to (1 ppm. If two elemental compositions are found within the mass tolerance, one formula could usually be confirmed/eliminated unequivocally by the presence/ absence of the corresponding nuclide containing one 13C. Because members of a homologous series differ only by integer multiples of CH2, assignment of a single member of such a series suffices to identify all members.41 We denote each elemental composition by its chemical formula, CcH2c+zX, (in which c is the carbon number, z is the hydrogen deficiency index, and X denotes the constituent heteroatoms (N, O, S) in the hydrocarbon molecule); thus, each additional double bond or ring increases the z value by 2 units. For example, in this notation, CcH2c-11N is designated as -11 N (see Table 2 for its list of Kendrick masses). Finally, since additional CH2 groups do not change the “class” or “type”, it is possible to determine a “carbon distribution” of all alkyl substituents for each class and type (see Discussion). On the basis of Kendrick mass analysis coupled with the accurate mass and high resolution afforded by high field FT-ICR MS, 465 of 479 ions from raw diesel fuel could be assigned to molecular formulas and 197 of 210 from processed diesel fuel. Above m/z 450, unequivocal identification became more difficult due to increased number of possible formulas within the mass tolerance. However, molecular formula assignment above m/z 450 proved unnecessary because those ions turned out to be dimers of lower molecular weight species (see Discussion). Finally, all species in the present mass spectra are singly charged, as evidenced by the 1 Da spacing between each monoisotopic species and its corresponding nuclide containing one 13C atom. Thus, from here (41) Hsu, C. S.; Qian, K.; Chen, Y. C. Anal. Chim. Acta 1992, 264, 79-89.

Figure 1. ESI FT-ICR mass spectrum of basic (B) and nonbasic (NB) model compounds. From left to right, the expected mass of each protonated model compound is indicated: oxindole (NB), 2-hydroxyquinoline (B), carbazole (NB), acridine (B), 1,10-phenanthroline (B), and 1-aminoanthracene (B). “Missing” compounds are those not ionized by electrospray.

on, we shall denote peaks by their mass in Da rather than as mass-to-charge ratio, m/z. Results and Discussion Electrospray Ionization of Basic and Nonbasic Nitrogen-Containing Compounds. To determine the extent to which basic (B) (pyridine homologues, primary amines) and nonbasic (NB) nitrogen-containing compounds (pyrrole, carbazole, indole homologues) are selectively ionized by electrospray, we electrosprayed an equimolar mixture consisting of oxindole (NB), 2-hydroxyquinoline (B), carbazole (NB), acridine (B), 1,10phenanthroline (B), and 1-aminoanthracene (B). Figure 1 shows the mass spectrum of the mixture, along with molecular structures for each compound. Only acridine and 1,10-phenanthroline were observed in the mass spectrum, indicating that basic pyridine homologues are selectively ionized by electrospray. Ionization of basic pyridine compounds was expected because lone-pair electrons on the nitrogen atom are not tied up in the π-electron cloud of the heterocyclic ring and are therefore available for protonation by acids (e.g., acetic acid in the electrospray solution). 2-Hydroxyquinoline is an exception. The presence and position of the hydroxyl group evidently decreases the nitrogen gas-phase basicity due to steric hindrance and hyperconjugation. Non-

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Figure 3. Expansion of an even-mass segment from Figure 2.

Figure 2. ESI FT-ICR mass spectra (260-440 Da) for unprocessed (top, 479 resolved peaks) and processed (bottom, 210 resolved peaks) diesel fuel. Inset: mass scale expansion from 320 to 328 Da. The most abundant species of even nominal mass are refractory compounds containing a single nitrogen; their corresponding nuclides containing a single 13C appear at odd mass. Asterisks denote members of the same Kendrick series, separated by intervals of 14.0156 Da. A peak list for this series is given in Table 2.

basic compounds are five-membered heterocycles in which lone pair electrons belonging to the nitrogen atom are delocalized around the aromatic ring and not available for donation to acids.7 Hence, nonbasic nitrogen compounds are not ionized by electrospray. (The peak at 163.9 Da in Figure 1 corresponds to 880 kHz, which is the operating frequency of one of our multipole ion guides.) Unprocessed versus Processed Diesel Fuel. At low displayed mass resolution, ESI FT-ICR mass spectra (260-440 Da) of unprocessed (Figure 2, top) and processed (Figure 2, bottom) diesel fuel exhibit approximately the same distribution and number of peaks. In fact, high resolution reveals 479 peaks for raw diesel fuel but only 210 peaks for processed diesel fuel over the same mass range. That difference alone suggests that various heteroatom (N, O, S) containing species are removed (i.e., decreased in concentration below the limit of detection of the mass spectrometer) by hydrotreatment. Closer scrutiny reveals unique patterns characteristic of petroleum samples. One group of peaks at 2.0157 Da intervals (i.e., the mass of two hydrogens) is seen in the mass scale-expanded inset, 320-328 Da. Peaks marked by asterisks are members of the same Kendrick/ homologous series, separated by intervals of 14.01565 Da, corresponding to the mass of one CH2 group. Similar patterns for electrosprayed diesel fuel have been reported by Zhan and Fenn and Miyabayashi, et al.31,32

Even- versus Odd-Mass Species and Removal of Heteroatom-Containing Species by Hydrotreatment. The “nitrogen rule” in mass spectrometry42 states that an even-electron ion of odd (even) mass contains an odd (even) number of nitrogen atoms. Nitrogencontaining positive ions generated by electrospray ionization are typically protonated even-electron species, [M + H]+. Thus, in the present examples, compounds containing an odd number of nitrogens appear at even mass, whereas compounds containing an even number of nitrogen atoms or one nitrogen and one 13C appear at odd mass. When viewed at nominal (i.e., unit) mass resolution (insets in Figure 2), the unprocessed and the processed diesel fuel mass spectra look almost identical because the most abundant components, pyridine homologues (and their corresponding nuclides containing one 13C instead of 12C), are resistant to hydrotreatmentsa conclusion noted in previous studies. Much more detail is revealed at high mass resolution, as seen in Figure 3 for species at a single nominal (even) mass. For example, the most abundant even-mass species in Figure 2 typically correspond to pyridine homologues (i.e., compounds containing one nitrogen within a six-membered aromatic ring), as seen at 324 Da in Figure 3. Additional NO-, NS-, and OS13C-containing compounds are observed in unprocessed diesel fuel (Figure 3, top). The NO and NS compounds are absent in the mass spectrum of processed diesel fuel. The NO and NS compounds may be removed by hydrotreatment (and therefore absent in the mass spectrum of processed diesel fuel) or might remain but with a loss of O or S. Because hydrotreating conditions are designed to remove sulfur-containing compounds, it is not surprising that NS-species are completely eliminated and/or converted. NO2-containing compounds comprising two minor series are seen only in the processed diesel mass spectrum. Each series begins above the mass shown in Figure 3 (380-450 and 340-382 Da). The NO2-containing compounds appear in the processed diesel but not in the unprocessed diesel (42) McLafferty, F. W.; Turecek, F. Interpretation of Mass Spectra, 4th ed.; University Science Books: Sausalito, CA, 1993.


Energy & Fuels, Vol. 15, No. 5, 2001 1191 Table 3. Compound Type Z Series Found in Unprocessed and Processed Diesel Fuela carbon no., c compound Z series

Figure 4. Expansion of an odd-mass segment from Figure 2, showing that OS-containing species can be protonated or sodiated. Compounds containing a single nitrogen and one 13C, as well as compounds containing two nitrogens, also appear at odd masses.

because their relative abundance increases (i.e., because other species are removed). NO2 compounds are an unlikely byproduct of hydrotreatment and are assumed to be refractory. Compounds typically observed at odd mass are shown at 343 Da in Figure 4 and primarily include protonated or sodiated OS-containing species. The presence of sodium adducts in fuel samples is controversial. One might expect instead that such compounds might contain two more carbons and one less hydrogen: 2C-H (22.992 Da) versus Na (22.9898 Da), a difference of 0.0022 Da. Although small, that difference is 2.7 times greater than the accuracy with which we can assign molecular formulas ((0.0004 Da or 1 ppm). Therefore, we are confident that sodium adducts are in fact present. Sodium adducts have also been observed for electrosprayed petroporphyrins.33 Although sodium adduct relative abundances vary from sample to sample, OS compounds, protonated or sodiated, are present in both unprocessed and processed diesel fuel mass spectra. Other compounds present at odd mass include N13C-, N2O-, N2-, NS13C-, and sodiated O2-containing species. N2O- and O2-containing compounds are evidently removed during hydrotreatment. N2-containing compounds are present in both the unprocessed and the processed diesel fuel although Z values vary. Higher Z values (-32N2 and -26N2) are found in unprocessed diesel fuel; lower Z values are found in processed diesel fuel (-22N2 and -20N2). There are two plausible explanations for those differences. First, the same species may be present in both fuels but the relative abundances of the “missing” Z series fall below the detection threshold (i.e., approximately 3 times the noise standard deviation). A similar explanation applies to differences in carbon number between unprocessed and processed fuels for a given compound class and Z type. Also, note that relative abundances necessarily vary from sample to sample due to low ionization efficiency: e1% of compounds in diesel fuel

raw diesel

processed diesel

CcH2c-31N CcH2c-21N CcH2c-19N CcH2c-17N CcH2c-15N CcH2c-13N CcH2c-11N CcH2c-9N CcH2c-7N

N Compounds 25-27 18-21 17-22 15-29 16-30 15-30 14-32 16-30 16-29

24-25 20-28 19-29 19-30 18-31 18-31 18-31 17-30

CcH2c-32N2 CcH2c-26N2 CcH2c-24N2 CcH2c-22N2 CcH2c-20N2

2N Compounds 29-31 26 23-27 -

24-30 26-28 22-27

CcH2c-23NS CcH2c-21NS CcH2c-17NS CcH2c-15NS CcH2c-13NS CcH2c-11NS CcH2c-9NS

N, S Compounds 21-22 17-20 16-24 13-24 18-21 15-26 14-23

-

CcH2c-29NO CcH2c-25NO CcH2c-19NO CcH2c-17NO CcH2c-15NO CcH2c-13NO CcH2c-11NO

N,O Compounds 23-24 25 17-25 17-22 16-22 16-18 14-16

-

CcH2c-24N2O CcH2c-22N2O CcH2c-20N2O

2N, O Compounds 23-24 23-24 21-22

-

CcH2c-17NO2 CcH2c-15NO2

N, 2O compounds -

25-29 22-25

S, O compounds 13-26 12-26 12-26

16-21 16-17 -

CcH2c-4SO CcH2c-2SO CcH2c-0SO a

Results from 200 to 452 Da.

are ionized by electrospray. (Table 1 shows that the elemental concentrations of N and S in the unprocessed diesel fuel are 370 ppm and 1.39 wt %, respectively, and processed diesel contains even less. But only polar (e.g., N-, S-, and O-containing) compounds (and only the most basic of those) will be observed by positive-ion electrospray (e.g., dibenzothiophenes43 are not seen). Only onethird of the nitrogen fraction is estimated to contain basic compounds.14) Second, N2-containing compounds, probably phenanthrolines, are partially hydrogenated but not removed by hydrotreatment. From the present results, we infer that compounds containing only nitrogen heteroatoms are more refractory to hydrotreatment than nitrogencontaining compounds which also contain either sulfur or oxygen heteroatoms. Results for unprocessed and processed diesel fuel are collected according to compound Z series and carbon distribution in Table 3. All (43) Rodgers, R. P.; Andersen, K. V.; White, F. M.; Hendrickson, C. L.; Marshall, A. G. Anal. Chem. 1998, 70, 4743-4750.

1192


Figure 5. Full-range ESI FT-ICR mass spectra of unprocessed (top) and processed (bottom) diesel fuel. High molecular weight species are dimers, as evidenced by their dissociation by infrared irradiation (see Figures 6 and 7).

heteroatom-containing species identified here have been identified previously by other researchers. Dimers. Although high molecular weight species (>450 Da) are observed in ESI FT-ICR mass spectra of both unprocessed and processed diesel samples (see Figure 5), they are much more abundant in the unprocessed sample. Hydroprocessing does not change the molecular weight distribution of fuels.7 Moreover, compounds in the present diesel samples have boiling points below 440 °C. One is thus led to suspect the presence of dimers (or other multimers). We therefore isolated ions above 450 Da by stored waveform inverse Fourier transform (SWIFT44,45) radial dipolar mass-selective ejection as shown in Figure 6 (top), followed by infrared multiphoton dissociation (IRMPD) by 40 W cw CO2 laser irradiation at maximum power for one second. Fragment ions are observed between 150 and 400 Da (Figure 6, bottom). The SWIFT-IRMPD mass spectrum was externally calibrated with PEG 200 and the fragmented ions classified as single N-containing and S,O-containing compounds. Kendrick series and masses of fragmented ions matched Kendrick series and masses in the original raw diesel fuel sample. Compounds left undissociated by IRMPD may be IR inactive. The formation of dimer ions in electrosprayed diesel fuel samples is concentration dependent. As described in the Experimental Section, both the unprocessed and processed diesel samples were prepared in the same manner to give a final concentration of ∼2.5% diesel fuel. However, the components are less concentrated in processed than in unprocessed diesel fuel due to partial removal by hydrotreatment. That is why more dimer peaks >450 Da are observed in the unprocessed than in processed diesel fuel (Figure 5). In support of that interpretation, doubling the concentration of the processed diesel fuel in the electrospray solution increased the number of above-threshold peaks above 450 Da as (44) Marshall, A. G.; Wang, T.-C. L.; Ricca, T. L. J. Am. Chem. Soc. 1985, 107, 7893-7897. (45) Guan, S.; Marshall, A. G. Int. J. Mass Spectrom. Ion Processes 1996, 157/158, 5-37.

Hughey et al.

Figure 6. ESI FT-ICR mass spectra of the SWIFT-isolated high molecular weight ions from raw diesel fuel before (top) and after (bottom) irradiation by a 40 W cw CO2 laser operated at maximum power for 1 s. (bottom) Fragment ions contain N or SO.

Figure 7. ESI FT-ICR mass spectra of ions from processed diesel fuel before (top) and after (bottom) IRMPD. The highmass species before IRMPD are believed to be SO-containing dimers, N-containing dimers, and dioctylphthalate-SO- or N-containing dimers, which are dissociated into monomers by IRMPD. The most abundant ion at 413.2668 Da corresponds to sodiated dioctylphthalatesa common contaminant from bottle cap linings.

shown in Figure 7 (top). Moreover, all dimer peaks are dissociated by IRMPD (Figure 7, bottom). Those dimers are believed to be S,O-containing and N-containing species, as in unprocessed diesel dimers. (The peak at 413.2668 Da in Figure 7 (bottom) corresponds to sodiated dioctyl phthalatesa common contaminant from bottle cap linings. Dioctyl phthalate has previously been found to form dimers with petroleum samples).33 SO-Containing Compounds. SO-containing compounds are interesting because their origin is unknown and their presence itself is surprisingssulfur compounds (e.g., benzothiophenes) do not electrospray, and organic sulfur is thus expected to be seen only in compounds also containing basic nitrogen. From the



Conclusion We have demonstrated the value of electrospray ionization combined with 9.4-T FT-ICR mass analysis for compositional analysis of a petroleum distillate. Electrospray selectively ionizes basic pyridine homologuesscompounds of interest because their presence in fuels hinders the removal of heteroatoms and promotes instability during storage. Prior to the present work, these compounds could be analyzed only after complex and inefficient separation schemes. Even after chromatographic isolation, instrumental limitations (i.e., lack of resolution and resolving power) have prevented the complete molecular analysis of complex petroleum mixtures. Figure 8. ESI FT-ICR MS spectrum of butyl sulfoxide before (top) and after (bottom) IRMPD.

mass spectra of both fuels, we find that SO compounds have the ability to form sodium adducts as well as SOSO dimers (and possibly SO-N dimers). Their low Z values (0-6) suggest more alkyl than aromatic character. We therefore propose that the SO-type compounds are sulfoxides. SO compounds have been previously reported in petroleum samples.21,25,26 We therefore electrosprayed two model compounds (dibutyl sulfoxide and diphenyl sulfoxide) as well as their corresponding sulfides (to ensure that oxidation to sulfoxides did not occurssulfides did not electrospray at all). Both sulfoxides behaved similarly to the SO compounds found in diesel fuel. Dibutyl sulfoxide, whose ESI FT-ICR mass spectrum is shown in Figure 8 (top), also forms sodium adducts and proton-bound dimers. Unlike SO-containing diesel compounds, dibutyl sulfoxide forms potassium adducts, sodium-bound dimers, and potassium-bound dimers. After IRMPD for 500 ms at full laser power (Figure 8, bottom), the hydrogenbound dimer mostly dissociates into its monomer. The relative abundances of the sodium-bound and potassium-bound dimers are unaffected by IRMPD, indicating stronger noncovalent bonds. Unidentified compounds in the unprocessed diesel fuel (Figure 6, bottom) are also left undissociated by IRMPD. However, the similarities between SO-containing compounds found in diesel fuel and sulfoxides do not unequivocally prove SO compounds to be sulfoxides, and more research is needed to determine the type and nature of SO-containing compounds.

Direct compositional analysis of raw diesel fuel and processed diesel fuel without prior chromatographic isolation is made possible by the high resolving power and mass accuracy afforded by FT-ICR MS. Use of Kendrick mass analysis simplifies molecular formula assignment of over 600 chemically distinct components. All 600 species contained nitrogen, oxygen, or sulfur. Compounds identified in unprocessed diesel fuel include N-, N2-, NS-, NO-, N2O-, O2-, and SO-containing molecules and in processed diesel fuel, N-, N2-, NO2-, and SO-containing species. Comparison of unprocessed and processed diesel fuel identifies N- and N2-containing compounds as resistant to laboratory hydrotreatment. NS-, NO-, N2O-, and O2-containing compounds are completely removed by hydrotreatment. Formation of dimers (>450 Da) was observed at high sample concentration. Acknowledgment. We thank Daniel McIntosh for machining all of the custom parts required for the 9.4 T instrument construction and John P. Quinn for many helpful discussions. We also thank Dr. Kuangnan Qian for helpful insight on use of the Kendrick mass system, and Dr. Kim Gron Knudsen from Haldor Topsoe A/S for providing the unprocessed and processed diesel samples. This work was supported by the NSF National High Field FT-ICR Facility (CHE-99-09502), Florida State University, and the National High Magnetic Field Laboratory in Tallahassee, FL. EF010028B

Elemental Composition Analysis of Processed and Unprocessed

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